linux-kernel - [PATCH 2/4] Pte clear optimization.patch

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Message-Id: <200704120530.l3C5Ulhr022828@zach-dev.vmware.com>
Date:	Wed, 11 Apr 2007 22:30:47 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Andrew Morton <akpm@...l.org>, Andi Kleen <ak@....de>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Chris Wright <chrisw@...s-sol.org>,
	Hugh Dickins <hugh@...itas.com>,
	David Rientjes <rientjes@...gle.com>,
	Michel Lespinasse <walken@...are.com>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Zachary Amsden <zach@...are.com>
Subject: [PATCH 2/4] Pte clear optimization.patch

When exiting from an address space, no special hypervisor notification of page
table updates needs to occur; direct page table hypervisors, such as Xen,
switch to another address space first (init_mm) and unprotects the page tables
to avoid the cost of trapping to the hypervisor for each pte_clear.  Shadow
mode hypervisors, such as VMI and lhype don't need to do the extra work of
calling through paravirt-ops, and can just directly clear the page table
entries without notifiying the hypervisor, since all the page tables are
about to be freed.

So introduce native_pte_clear functions which bypass any paravirt-ops
notification.  This results in a significant performance win for VMI
and removes some indirect calls from zap_pte_range.

Note the 3-level paging already had a native_pte_clear function, thus
demanding argument conformance and extra args for the 2-level definition.

Signed-off-by: Zachary Amsden <zach@...are.com>

diff -r 1478ce4ec9e3 include/asm-i386/pgtable-2level.h
--- a/include/asm-i386/pgtable-2level.h	Wed Apr 11 17:13:10 2007 -0700
+++ b/include/asm-i386/pgtable-2level.h	Wed Apr 11 18:22:51 2007 -0700
@@ -35,6 +35,11 @@ static inline void native_set_pmd(pmd_t 

 #define pte_clear(mm,addr,xp)	do { set_pte_at(mm, addr, xp, __pte(0)); } while (0)
 #define pmd_clear(xp)	do { set_pmd(xp, __pmd(0)); } while (0)
+
+static inline void native_pte_clear(struct mm_struct *mm, unsigned long addr, pte_t *xp)
+{
+	*xp = __pte(0);
+}

 static inline pte_t native_ptep_get_and_clear(pte_t *xp)
 {
diff -r 1478ce4ec9e3 include/asm-i386/pgtable.h
--- a/include/asm-i386/pgtable.h	Wed Apr 11 17:13:10 2007 -0700
+++ b/include/asm-i386/pgtable.h	Wed Apr 11 18:21:43 2007 -0700
@@ -349,7 +349,7 @@ static inline pte_t ptep_get_and_clear_f
 	pte_t pte;
 	if (full) {
 		pte = *ptep;
-		pte_clear(mm, addr, ptep);
+		native_pte_clear(mm, addr, ptep);
 	} else {
 		pte = ptep_get_and_clear(mm, addr, ptep);
 	}
-
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